1. Field of the Invention
The present invention relates to a document reading apparatus configured to detect the size of a document set on a document positioning plate (platen).
2. Description of the Related Art
Conventionally, various document reading apparatuses are developed as apparatuses that are useful in detecting the size of a document that is set on a document positioning glass plate.
Japanese Patent Application Laid-Open No. 5-207239 discusses an apparatus that detects the size of a document using a charge-coupled device (CCD) and a document presence/absence sensor. This apparatus illuminates a document with a light beam emitted from a light source. Then, according to an output of the CCD, which receives light reflected from the document, the apparatus detects the length of the document in the main scanning direction. The main scanning direction corresponds to a direction in which sensor elements of the CCD are arrayed. Further, the apparatus can discriminate between documents having the same length in the main scanning direction but having different sizes based on an output of the document presence/absence sensor. In other words, the apparatus detects a document size based on the detected length of the document in the main scanning direction and the output of the document presence/absence sensor.
Since the length in the main scanning direction is detected by the CCD used for document reading and the length in the sub-scanning direction, which is perpendicular to the main scanning direction, is detected by one or a small number of document presence/absence sensors, efficient document size detection can be achieved at low cost.
However, if ambient light, such as interior light or sunlight, is incident on the document positioning glass plate from an area where the document is absent and the light quantity of the ambient light equals or is larger than the light reflected by the document, a detection error of the document size may occur.
To prevent such a detection error from occurring, Japanese Patent Application Laid-Open No. 2001-346009 discusses an apparatus that detects an area where the CCD outputs are present based on the CCD outputs that are obtained at the time the light source is turned off, and then detects the length of a document in the main scanning direction based on a signal output from the CCD in an area excluding the detected area at the time the light source is turned on.
However, if strong ambient light is incident on the document positioning glass plate on which a document that tends to transmit light, such as thin paper, is set, then even if the apparatus discussed in Japanese Patent Application Laid-Open No. 2001-346009 is used, the ambient light may pass through the document, and the area where the document is actually present may be regarded as not present, and as a result, a detection error may occur.